1. Field of the Invention
The present invention relates to a method of rolling a wide flange beam by the use of a universal rolling mill comprising horizontal rolls and vertical rolls. More particularly, the present invention relates to a method of rolling, in a universal rolling mill, a wide flange beam excellent in accuracy of a web thickness and a flange thickness by causing a drive of the screw-down for the horizontal rolls and a drive of the screw-down for the vertical rolls to operate independently of each other by means of measured values of rolling load acting on the horizontal rolls and the vertical rolls during rolling, even with an unknown rigidity of the material to be rolled.
2. Description of the Related Art
There has conventionally been well known a technique of controlling the thickness of a flat sheet by causing a change in the rolling roll gap during rolling. As is described in the "Theory and Practices of Plate and Sheet Rolling" edited by the Iron and Steel Institute of Japan, p. 229-231, a basic technique of measuring a rolling load with a load cell or the like, and causing a change in the roll gap in accordance with the following formula, thereby controlling the sheet thickness is known as the gauge meter AGC (Automatic Gauge Control): EQU .DELTA.S=-.alpha..DELTA.F/K.sub.m ( 1)
where, .DELTA.S is an amount of change in the roll gap, .DELTA.F is a change in load from a lock-on load, K.sub.m is a mill rigidity, and .alpha. is a correction coefficient known as a tuning rate.
Operation of the roll gap is performed by the use of an electrically driven screw-down motor or a hydraulic cylinder, and it is more often the common practice to use a hydraulic cylinder because of a better response and benefits in mechanical structure.
By the use of the foregoing gauge meter AGC, a variation in flat sheet thickness .DELTA.h can be calculated by the following formula: ##EQU1## Since the load .DELTA.F varies also with a variation in thickness, it can be expressed by the following formula: EQU .DELTA.F=Q.DELTA.h+.DELTA.F.sub.dis ( 3)
where, Q is rigidity (plasticity constant) of the flat sheet, and .DELTA.F.sub.dis is a load disturbance caused by a variation in thickness on the entry side or a change in rolling temperature. From the foregoing formulae (2) and (3), the variation in flat sheet thickness .DELTA.h can be expressed eventually by the following formula: EQU .DELTA.h={(1-.alpha.)/(K.sub.m +(1-.alpha.)Q}.DELTA.F.sub.dis ( 4)
where, it is known that the tuning rate .alpha., generally taking a value near 1, an error, if any, in flat sheet thickness .DELTA.h can be minimized even upon occurrence of a load disturbance .DELTA.F.sub.dis.
While the formulae (1) to (4) have shown stationary properties of gauge meter AGC, a transient response depends upon dynamic properties of the drive.
Now, the features of the gauge meter AGC will be briefly described below. As is described in the aforesaid reference, the gauge meter AGC has transient dynamic properties varying with rigidity (plasticity constant) of the material. However because the tuning rate generally takes a value near 1, the stationary properties do not depend upon rigidity Q or deformation property of the material, as shown in the formula (4). This control is achievable by only knowing the easily predictable mill rigidity K.sub.m even when rigidity Q of the material difficult to predict in general is unknown, as indicated by the formula (1). This is a feature of the gauge meter AGC.
The difficulty in predicting a material rigidity is caused by plastic deformation of the material. Because plastic deformation largely depends upon material quality and temperature, it is difficult to predict this phenomenon. Since a mill complies with elastic deformation, on the other hand, prediction of deformation thereof is easy.
The foregoing gauge meter AGC is widely applied to flat rolling mills. Because of the difficulty in mechanical structure, however, there are available only a few cases of application of the gauge meter AGC to a universal rolling mill for rolling a wide flange beam having a web portion 12 and flange portions 14 as shown in FIG. 6. However, in accordance with the same technique as that of a flat rolling mill, it suffices to conduct rolling as follows: EQU .DELTA.S.sub.h =-.alpha..DELTA.F.sub.w /K.sub.mh ( 5) EQU .DELTA.S.sub.v =-.alpha..DELTA.F.sub.f /K.sub.mv ( 6)
where, h is a horizontal roll, v is a vertical roll, .DELTA.F.sub.w is a rolling load of the web portion 12 and .DELTA.F.sub.f is a rolling load of the flange portion 14.
It is estimated, as in the case of flat rolling, that this control permits reduction of errors in web and flange thickness.
As described above, application of the gauge meter AGC to a universal rolling mill can easily be conceived. An important difference from flat rolling is however that the web portion 12 and the flange portions 14 in a wide flange beam 10 are connected, and rolling of the web portion 12 and the flange portions 14 mutually exerts an effect. For example, when the vertical roll gap is tightened to reduce the thickness of the flange portions 14, the load acting on the horizontal rolls is known to be reduced. Therefore, independent application of the gauge meter AGC to vertical roll rolling and horizontal roll rolling as described above would result in a serious mutual influence of the web portion and the flange portions, hence causing interference between the vertical rolls and the horizontal rolls and leading to undesirable vibration.
FIG. 5 illustrates a structure of a universal rolling mill 20 in a case where roll screw-down is performed by means of a hydraulic cylinder. In FIG. 5, horizontal rolls 22 reduce from above and below the web portion 12 of a wide flange beam, and vertical rolls 24 reduce from right and left the flange portions 14 of the side flange beam.
The horizontal rolls 22 are provided, for example, with hydraulic cylinders 26 and 28 for the upper horizontal roll for screwing down at right and left ends of the upper horizontal roll shaft, and hydraulic cylinders 30 and 32 for the lower horizontal roll are provided for a similar purpose for the lower horizontal roll 22. The vertical rolls 24 are similarly provided, for example, with hydraulic cylinders 34 and 36 for the left vertical roll for screwing down the left vertical roll from front and back thereof, and hydraulic cylinders 38 and 40 for the right vertical roll for screwing down similarly the right vertical roll from front and back thereof. These hydraulic cylinders are arranged above and below, and at right and left because the entire universal mill must form a point-symmetry for rolling a wide flange beam.
In order to apply the gauge meter AGC, it is necessary to measure the load during rolling. A load cell is provided for each drive for this purpose. More specifically, as shown in FIG. 5, the right and left hydraulic cylinders 26 and 28 for the upper horizontal roll are provided with load cells 42 and 44 for the upper horizontal roll, respectively, and the right and left hydraulic cylinders 30 and 32 for the lower horizontal roll are provided with load cells 46 and 48 for the lower horizontal roll, respectively. The front and rear hydraulic cylinders 34 and 36 for the left vertical roll are provided with load cells 50 and 52 for the left vertical roll, respectively, and the front and rear hydraulic cylinders 38 and 40 for the right vertical roll are provided with load cells 54 and 56 for the right vertical roll, respectively.
FIG. 7 illustrates a common control configuration of the gauge meter AGC based on hydraulic cylinder screw-down popularly applied in flat plate, cold or hot rolling. In FIG. 7, 60 is a load cell, showing a load F provided as an output. The portion enclosed by dotted lines represents a controller or arithmetic unit 68. The portion within the dotted lines shows a computing logic in the arithmetic unit. The arrows represent the flow of signals, and the symbol on the arrow, the value of signal. The symbol +/- on or to the left of the arrow means addition/subtraction of the value of signal. The squares within the dotted lines means that an input signal from the left is multiplied by a parameter shown by a signal in the square and a resultant signal is issued as an output. A servo valve 62 is adjusted by means of a final output signal from the arithmetic unit 68 to move a cylinder 64 through a hydraulic piping 66. A cylinder positional signal S.sub.FBK is fed back to the arithmetic unit 68. Among the signals within the dotted lines, .DELTA.F represents a deviation from the lock-on load F.sub.o, K.sub.m is a mill constant, .alpha. is a tuning rate, .DELTA.S is an AGC control amount, S.sub.o is a (hydraulic) cylinder positioning value before biting, S.sub.FBK is a measured value of cylinder position, and G is a cylinder position control gain. The positioning time must be adjusted so that the cylinder positioning time before biting does not become excessively longer, since the positioning time depends upon this control gain G. The control gain G is therefore usually adjusted so as to ensure execution of cylinder position setting at the highest possible speed, while observing a response of the hydraulic cylinder 64 and the like.
FIG. 8 illustrates a case of independent application of the flat rolling gauge meter AGC shown in FIG. 7 to horizontal rolling and vertical rolling on a universal mill 20. The upper portion relative to a one-point chain line corresponds to horizontal rolling, and the lower portion, to vertical rolling. In this thickness controller, the gauge meter AGC apparatus 70 based on screw-down by the hydraulic cylinder of the horizontal roll and the gauge meter AGC apparatus 72 based on screw-down by the hydraulic cylinder of the vertical roll are independent of each other. In FIG. 8, F.sub.h is a load acting on the horizontal roll 22, .DELTA.F.sub.h is a deviation from the horizontal roll lock-on load F.sub.h0, K.sub.hm is a mill constant in the vertical direction of the universal mill 20, .alpha. is a tuning rate, .DELTA.S.sub.h is a horizontal roll AGC control amount, S.sub.h0 is a set value of the horizontal roll cylinder position before biting, and G.sub.hi (i=1 to 4) is a positional control gain for each cylinder. Similarly, variables such as F.sub.v, .DELTA.F.sub.v, F.sub.v0, .DELTA.S.sub.v and G.sub.vi (i=1 to 4) are defined also for the vertical roll 24. K.sub.vm is a mill constant in the transverse direction of the universal mill 20. Usually, the cylinder positional control gains G.sub.hi and G.sub.vi must be adjusted so as to ensure rapid positional setting before biting.
FIG. 9 illustrates the result of control in a case of application of the controller shown in FIG. 8. As is clear from FIG. 9, as a result of mutual influence of the web and the flanges of a wide flange beam occurring during rolling, a behavior suggesting vibration appears immediately upon start of control, resulting in a large thickness deviation.
To solve this problem, there is proposed a method of eliminating interference taking account of the mutual influence when controlling the flange portions and the web portion, as described in the "Non-Interference Thickness Control of Large-Scale Rolling Mill, Iron and Steel-Making Research, No. 317 (1985), p. 48-58" and Japanese Examined Patent Publication No. 63-66608. More specifically, by the use of a linearized rolling load model describing the mutual influence of the web portion and the flange portions during rolling, the proposed method comprises the steps of operating drives so as to prevent mutual interference, thereby eliminating the interference phenomenon.
In such a method of control for eliminating interference between the horizontal rolls and the vertical rolls by means of a rolling load model, it is necessary to provide a model strictly describing the rolling phenomenon, and the following problems have been posed.
(1) In the wide flange beam rolling presenting a three-dimensional deformation property, it is very difficult to prepare a model strictly describing the rolling phenomenon because of the difficulty in applying the well-known rolling theory.
(2) In general, a plastic deformation phenomenon is a non-linear model, and it is difficult to completely eliminate interference because of this non-linearity.
(3) A driving equipment generally exhibits a high-order response, and it is difficult to ensure elimination of interference with the high-order response also in view.
Even with an unknown rigidity of the material difficult to predict in general, a feature of the flat rolling gauge meter AGC is to permit achievement thereof by knowing only the mill rigidity. However, simple application of the flat rolling gauge meter AGC to a universal mill causes the problem as described above.